Electroless copper plating solutions



United States Patent US. Cl. 106-1 Claims ABSTRACT OF THE DISCLOSURE Highly stable electroless copper solutions are obtained by incorporating thereto small quantities of a complexing agent having the empirical formula wherein A is a sodium, potassium, lithium or copper atom, R and R are hydrogen or straight or branched chain radicals having from 1 to 6 carbon atoms, and X is nitrogen.

The present invention is related to electroless plating solutions and, more particularly, it concerns electroless copper solutions which are particularly resistant to rapid decomposition.

Generally, electroless copper solutions are either Fehlings solutions or modifications thereof. They, however, contain incorporated therein a reducing agent such as formaldehyde. It is known that the above-mentioned solutions begin to decompose Within about twenty minutes after make-up and, ordinarily, cannot be utilized for deposition purposes after about three hours.

It is also known to add to these solutions various quantities, usually quite substantial, of complexing agents, such as, for example, Rochelle salts, Versene, and the like. However, the resulting stability of the solutions and the rapid rate at which deposit formation occurs leave much to be desired.

It is, therefore, the main object of the present invention to provide a highly stable electroless copper solution, superior to those mentioned hereabove.

It is a further object of this invention to provide a novel solution which requires only very small quantities of a complexing agent to achieve highly stable solutions.

Other objects and advantages of the invention will become apparent from the following detailed description thereof.

It has been found that minor additions of a compound having the empirical formula wherein X is nitrogen, A is a sodium, potassium, lithium, or copper atom, and R and R are hydrogen or straight and/or branch chained radicals having from 1 to 6 carbon atoms to diluted Fehlings solution and formaldehyde results in a clearly remarkable increase in the stability of the electroless copper solution.

The amount of the above compound added to sufiiciently stabilize the solution ranges from about 0.05 to milligrams of compound per liter of Fehlings solution, to which formaldehyde is to be added. The resultant solution has been found to allow highly satisfactory depositions at rates faster than any other tested known copper solutions and yet with a stability remarkably increased.

This invention will become even more apparent from the following examples.

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A Fehlings solution was prepared as specified, which consisted of 34.64 g./l. of CuSO -5H O, 173 g./l. of Rochelle salt 4H O, 50 g./l. of NaOH, and the balance distilled water. Certain aliquots of solutions were mixed with 36% formaldehyde (containing 12.5% of methanol as a preservative) in quantity such as to give various solution concentrations. To other aliquots of Fehlings solution were added varying concentrations of selected compounds of the invention and, subsequently, varying quantities of the above-mentioned 36% formaldehyde. Comparative tests between a solution containing an additive of the invention and a solution free from such additive were conducted, with the following tabulated results:

EXAMPLE 1 (1) Solution concentration without additive (percen 100 (2) Additive, mg./l. (sodium dlhexyl-dithio carbamate 0 20 (3) Solution temperature C.) 25 25 (4) Deposit weight (mg/0.25 hr.) 49 39 (5) Bath stability Poor Fair (8) Deposit appearance Poor Good (7) Deposition after 3 hours (mg/0.25 hr.) 29

EXAMPLE 2 (1) Solution concentration without additive (percent) 100 100 (2) Additive, mg./l. (potassium dibutyl-dithio carbarnate) 0 0. 05 (3) Solution temperature C.) 25 25 (4) Deposit weight (mg/0.25 hr.) 49 35 (5) Bath stability Poor Poor (6) Deposit appearance Poor Fair (7) Deposition after 3 hours (mg/0.25 hr.) 0 24 EXAMPLE 3 (1) Solution concentration without additive (percen 50 50 (2) Additive, mg./1. (copper diethyl-dithio carbainate 0 1 (3) Solution temperature C.) 25 25 (4) Deposit weight (mg/0.25 hr.). 36 31 (5) Bath stability Poor Good (6) Deposit appearance Poor Good (7) Deposition after 3 hours (mg/0.25 hr.) 26

EXAMPLE 4 (1) Solution concentration without additive (percent) 50 50 (2) Additive, mg./l. (lithium dimethyldithio carbamate) 0 5 (3) Solution temperature 0.). 25 25 (4) Deposit weight (mg/0.25 hr. 36 3G (5) Bath stability Poor Poor (6) Deposit appearance Poor Fair (7) Deposition after 3 hours (mg/0.25 hr.) 28

EXAMPLE 5 (1) Solution concentration without additive (percen 25 25 (2) Additive, mg./l. (sodium dunethyl-dithio carbamate 0 0. 25 (3) Solution temperature C.) 25 25 (4) Deposit weight (mg/0.25 hr. 23 5 (5) Bath stability Fair Good (6) Deposit appearance Good Good (7) Deposition after 24 hours (mg./ 0.25 hr.) 3

EXAMPLE 6 (1) Solution concentration without additive (percent) 25 25 (2) Additive, mgJl. (sodium (liethyl-dithio carbamate) 0 4 (3) Solution temperature 0.). 25 25 (4) Deposit weight (mg/0.25 hr. 23 20 (5) Bath stability Fair Good (6) Deposit appearance Good Good (7) Deposition after 24 hours (mg/0.25 hr.) 0 15 With reference to the above examples, it should be understood that poor bath stability is one in which the bath is substantially decomposed within about 3 hours, a fair bath stability is one which is relatively undecomposed within 3 hours but almost completely decomposed in 24 hours; and a good bath stability shows little or no decomposition after 24 hours.

A poor deposit appearance is a non-uniform or powdery deposit, while a good deposit appearance is very coppery metallic. An intermediate appearance is rated, obviously, as fair. The deposition tests were conducted on 2 square inch, cleaned and sensitized Mylar sheet of 3 mil thickness.

From the above examplary tests, it is conclusive that, in almost every single instant, a very small addition of one of the compounds of the present invention has contributed definitely to achieve an improvement in the deposition rate and stability of the bath. The main advantage of the present invention is an electroless copper solution which is usable for at least several days, thus obviating the necessity for time-consuming make-ups.

What is claimed is:

1. An electroless copper plating solution consisting essentially of Fehlings solution, a reducing agent, and a stabilizing amount of a compound having the empirical formula II R: S

wherein A is a sodium, potassium, lithium or copper atom; X is nitrogen; and R and R are hydrogen or straight or branched chain radicals having from 1 to 6 carbon atoms.

2. The electroless copper plating solution of claim 1, wherein said reducing agent is formaldehyde.

3. The electroless copper plating solution of claim 2, wherein said stabilizing compound is present in amounts of from about 0.05 to 20 milligrams per liter of undiluted Fehlings solution.

4. The electroless copper plating solution of claim 1, consisting essentially of Fehlings solution, a reducing agent, an additional volume of water from 1 to 3 times the volume of undiluted Fehlings solution, and from 0.05 to 20 mg./liter of undiluted Fehlings solution of sodium dihexyl-dithio carbamate.

5. The electroless copper plating solution of claim 1, consisting essentially of Fehlings solution, a reducing agent, an additional volume of water from 1 to 3 times the volume of undiluted Fehlings solution, and from 0.05 to 20 mg./liter of undiluted Fehlings solution of potassium dibutyl-dithio carbamate.

6. The electroless copper plating solution of claim 1, consisting essentially of Fehlings solution, a reducing agent, an additonial volume of water from 1 to 3 times the volume of undiluted Fehlings solution, and from 0.05 to 20 mg./liter of undiluted Fehlings solution of copper diethyl-dithio carbamate.

7. The electroless copper plating solution of claim 1, consisting essentially of Fehlings solution, a reducing agent, an additional volume of water from 1 to 3 times the volume of undiluted Fehlings solution and from 0.05 to 20 mg./liter of undiluted Fehlings solution of lithium dimethyl-dithio carbamate.

8. The electroless copper plating solution of claim 1, consisting essentially-of Fehlings solution, a reducing agent, anadditional volume of wat r from 1 to 3 times the volume of undiluted Fehlings so ution, and from 0.05 to 20 mg./liter of undiluted Fehlings solution of sodium dimethyl-dithio carbamate.

9. The electroless copper plating solution of claim 1, consisting essentially of Fehlings solution, a reducing agent, an additional volume of water from 1 to 3 times the volume of undiluted Fehlings solution, and from 0.05 to 20 mg./liter of undiluted'Fehlings solution of sodium diethyl-dithio carbamate.

10. A process for improving the stability of an electroless copper plating solution consisting essentially of Fehlings solution and a reducing agent, which comprises adding a stabilizing amount of a compound having the empirical formula References Cited UNITED STATES PATENTS 1/1968 Schneble et al. 1061 4/1968 Torigai et al. 106-1 JULIUS FROME, Primary Examiner. L. HAYES, Assistant Examiner.

US. Cl. X.R. 1l7130 

